Daniel is planning to start growing his own bell peppers. He checks a few references about the type of fertilizer the plants should receive in order to produce the maximum number of peppers, and he reads that phosphorus is important for fruit production. So, Daniel decides to test this hypothesis. Daniel buys five of the same species of bell pepper plants in similar pots, and he places all of them in the same area of his patio, in full sunshine. He numbers each pot 1-5 and adds fertilizer according to the number: Pot #1 receives 20 g fertilizer with 0% phosphorus. Pot #2 receives 20 g fertilizer with 20% phosphorus. Pot #3 receives 20 g fertilizer with 40% phosphorus. Pot #4 receives 20 g fertilizer with 60% phosphorus. Pot #5 receives 20 g fertilizer with 80% phosphorus. Daniel counts the number of new peppers produced by each plant each week for eight weeks. What is a constant in Daniel's experiment?